Canine & Feline Heartworm Flashcards
what is the nematode that causes heartworm infection
dirofilaria immitis
how is dirofilaria transmitted to dogs and cats
mosquito vector
what species are more commonly affected by heartworm disease
dogs > cats
cats are atypical hosts
where is heartworm endemic
all lower 48 states
where is heartworm hyperendemic
SE states and mississippi river valley
what drugs are used for heartworm prevention in dogs and cats
macrocyclic lactones (ML)
ivermectin, selamectin, moxidectin, milbemycin
what life stage of the heartworm lifecycle do macrocyclic lactones target
kills L3 and early L4 larvae
does NOT kill adults
when should a puppy start using prevention
6 to 8 weeks old
use year round
heartworm life cycle in a dog
- mosquito ingests microfilaria from a HW infected host
- in the mosquito - dirofilaria grow to L3 larvae in 10 days
- mosquito bites a new dog and transmitted L3 larvae into the dog tissues
- L3 larvae grows into L4 and resides in tissues (45-65 days)
- L4 enters the bloodstream and develops into an adult (4-5 months)
- adults in the blood start producing microfilaria
how long post-infection does microfilaria production begin in dogs
6 to 7 months
how long do adult HWs and microfilaria live in dogs
adults - 5-7 years
microfilaria - 1-2 years
heartworm lifecycle in a cat
- mosquito ingests microfilaria from a HW infected host
- in the mosquito - dirofilaria grow to L3 larvae in 10 days
- mosquito bites a new cat and transmits L3 larvae into the cat tissues
- L3 larvae grows into L4 and resides in tissues (2 months)
- cat immune system kills majority of larvae and developing adults - L4 enters the bloodstream and develops into an adult (4-6 months)
- adults in the blood start producing microfilaria
how long post-infection does microfilaria production begin in cats
7-8 months
do microfilaria circulate in cats
rarely
where do adult heartworms reside in dogs
pulmonary arteries
majority in the caudal pulmonary vascular tree
what does the severity of clinical signs depend on
- relative # of worms
- duration of infection
- host-parasite interaction
how does the cat immune system respond to heartworm
many immature adults die once they reach the pulmonary arteries
infection is typically cleared at the L4/early L5 stage
even if cleared - can still cause inflammation in the pulmonary parenchyma and bronchioles –> cough, tachypnea
what is the typical worm burden in cats
1-3 worms
causes many false negatives on antigen tests (threshold is 3 worms)
what is wolbachia
gram negative bacteria
gets released in large numbers during worm death and exacerbate acute inflammatory response + increased bronchoreactivity
what can occur in feline HW infections that doesn’t occur in canine infections
aberrant migrations - neurologic, ocular, arterial
what heartworm test is used in dogs
antigen testing
high sensitivity and specificity - good screening test
what does the antigen test detect and how early can it detect it after infection
adult females
must be at least 3 worms
detects antigen 6 months after infection
- earliest detectable time for puppies - 8 months old
if a dog has a lapse in prevention for > 6 months, when should they be tested
now and again in 6 months
if a dog has a lapse in prevention for <6 months, when should they be tested
6 months after re-starting prevention
how often should heartworm screening be performed
yearly
what is microfilaria testing
tests for the presence of microfilaria in the blood
variable sensitivity - mostly used as a safety net to catch false negative antigen tests
what heartworm test is used in cats
antibody test
detects antibodies against antigens produced by both male and female larvae
why can the antigen test not be used in cats
cats often have too small of an adult worm burden (1-3 worms) and results in high false negatives
if a cat is positive on an antigen test - guarantee the cat is positive
when should radiographs be used in cats suspected of HW
cats with respiratory signs
- pulmonary pattern
- pulmonary artery enlargement
- R sided heart enlargement
what pathology is associated with heartworm disease in dogs
pneumonitis - inflammation of the pulmonary parenchyma + airways
vasoconstriction and occlusion of the lumen of the pulmonary arteries
what pathology is associated with heartworm disease in cats
significant pathology even in aborted or cleared disease
- pulmonary arteries
- pulmonary alveoli + parenchyma
- bronchi, bronchioles
clinical signs of heartworm disease in dogs
majority are ASYMPTOMATIC or have mild cough
- exercise intolerance
- severe cough or respiratory distress
- syncope
- hemoptysis
- R-CHF from pulmonary hypertension
- caval syndrome
clinical signs of heartworm disease in cats
larval infection: subclinical OR cough, tachypnea, dyspnea, ARDS
adult infection: self clearance (no signs) OR GI signs, respiratory signs, R sided murmurs, syncope, ARDS, sudden death
how to treat pneumonitis from heartworm disease in dogs
- O2 supplementation
- steroids
- dexSP (IV)
- prednisolone (PO)
how to treat heartworm disease in dogs
- doxycycline
- 2 to 3 dose melarsomine
- steroid taper after each melarsomine dose
ACTIVITY RESTRICTION
what is the role of doxycycline in heartworm treatment
kills wolbachia that’s released by the worms
what is the role of prednisone taper after melarsomine injections
reduces inflammation from worm death that could otherwise cause thrombosis + vascular occlusion
melarsomine
kills adult L5 worms
why is activity restriction important
want to avoid exacerbating pulmonary vascular and parenchymal pathology
can lead to worsening pneumonitis and pulmonary hypertension
how to treat heartworm disease in cats
do NOT use melarsomine
adulticidal:
- doxycycline + ivermectin
- surgical HW extraction
at home tx:
- prevention
- prednisolone
- fluticasone inhaler
- albuterol inhaler
- +/- clopidogrel
how to treat cats with severe respiratory distress from HW
- O2 supplementation
- corticosteroids (dexamethasone)
- bronchodilators
what are the complications of severe heartworm disease in dogs
- pulmonary hypertension
- pulmonary thromboembolism
- R-CHF (from PH)
- caval syndrome
how to diagnose and treat pulmonary hypertension
echocardiogram - tricuspid regurgitation velocity
treat with sildenafil
how to diagnose and treat PTE from heartworm disease
supportive diagnosis - thoracic radiographs with signs of HWD and PH
treat with steroids +/- clopidogrel
caval syndrome
chronic pulmonary hypertension + reduced RV function –> retrograde movement of heartworms into the RA + vena cava –> obstructs venous return –> R-CHF and reduced CO
clinical signs of caval syndrome
lethargy
tachypnea
syncope
ascites
discolored urine (pigmenturia)
treatment of caval syndrome
EMERGENCY
transjugular HW extraction
how to treat R-CHF from heartworm disease in cats
O2 supplementation
sedation
thoracocentesis
furosemide
abdominocentesis
